Anyone use #300 main jets on a MK3?

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Have you checked the breather on the petrol tank ? Fuel starvation usually gives erratic running. A plug chop using plugs which are one heat range hotter than normal, will usually keep you safe - as long as the black ring remains on the porcelain, right down inside the plug where it meets the metal.
 
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acotrel said:
Have you checked the breather on the petrol tank ? Fuel starvation usually gives erratic running. A plug chop using plugs which are one heat range hotter than normal, will usually keep you safe - as long as the black ring remains on the porcelain, right down inside the plug where it meets the metal.
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the plugs look like new, the cleanest I have ever seen
I will recheck the timing
 
I have used the slotted float bowls for racing.

The bike would run perfectly but start to miss at full throttle at the end of a 1 km straight. Going through the fuel delivery lines from start to finish including slotting the bowls solved the problem. So I cannot be certain which change did the trick but collectively it worked. This was on a 500 with 200 main jets. Mk 1 Amals.

And yes In my opinion something else is the issue. 300 mains are too big.
 
Jimbo, unless your engine is heavily modified, something else may be at play, maybe a vacuum leak?
The largest I've ever run is a 270 on stock 850 with 2-1-2 Dunstalls, and I'm at sea level.
280 was too big. Can't imagine a 300.

But then again, go ahead and try it. If that's what it is, that's what it is..
 
jimbo said:
wow, how do you explain that?
When I had the bike WOT ,then closed the throttle just a little , it seemed to pull harder, that's why I think I need more jet
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I had that exact problem too at about 80mph... Because of the speed at which it occurred and how well my bike ran in every other range, I thought that the obvious place to check was main jet size. In my case, backing off slightly made it speed up and the fix was a smaller main jet. I was running 240 mains (on 930 amals) and the proper spec'd main size was 220. As it turns out, with the 220's in place my bike flys right past 80mph now...
 
MichaelB said:
Jimbo, unless your engine is heavily modified, something else may be at play, maybe a vacuum leak?
The largest I've ever run is a 270 on stock 850 with 2-1-2 Dunstalls, and I'm at sea level.
280 was too big. Can't imagine a 300.

But then again, go ahead and try it. If that's what it is, that's what it is..
Click to expand...
I will look over everything else again, but it sounds as though a 300 main is too big for anything I have


Here are my plugs on 260's, don't look too much different with the 280's


Anyone use #300 main jets on a MK3?
Anyone use #300 main jets on a MK3?
Anyone use #300 main jets on a MK3?
 
o0norton0o said:
I had that exact problem too at about 80mph... Because of the speed at which it occurred and how well my bike ran in every other range, I thought that the obvious place to check was main jet size. In my case, backing off slightly made it speed up and the fix was a smaller main jet. I was running 240 mains (on 930 amals) and the proper spec'd main size was 220. As it turns out, with the 220's in place my bike flys right past 80mph now...
Click to expand...


interesting ... why are my plugs so clean?

I guess I could try leaner
 
jimbo said:
I checked the pingel(s) they flow fine. tank cap breather is clear. Just wondered if 300's were unusual
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Was looking at pingel taps for a ‘71 Sportster. Wow those are expensive. They must work really well.
 
You may need to go richer on the needle jet. [or needle] The needle jet may be the full throttle restriction once the main is that large. If your running the MK3 needle that was designed for the full airbox that may be the problem.

Since you lowered the compression it would be normal to need larger jets.
 
comnoz said:
You may need to go richer on the needle jet. [or needle] The needle jet may be the full throttle restriction once the main is that large. If your running the MK3 needle that was designed for the full airbox that may be the problem.

Since you lowered the compression it would be normal to need larger jets.
Click to expand...
not sure what needle I have, its sitting at the top( clip in the lowest groove) now. I think I bought new needles and needle jets a while ago, I will look into it.
 
jimbo said:
interesting ... why are my plugs so clean?

I guess I could try leaner
Click to expand...

When I had my issue, which has similar sounding symptoms to yours. I changed down to 220's from the 240's, but went out riding my test loop with a jet key and the 260's in my pocket in case I had guessed wrong about my issue and went further away from the proper jet size.

I'm not talking out my ass theoretically. I'm just reporting my actual experience with similar symptoms, and reporting that my fix was the smaller stock jet size that eliminated the wall I hit at 80mph...

read what I wrote in 2015:

https://www.accessnorton.com/NortonCommando/750-combat-top-speed-problem-2015.18379/page-4

I'm also a proponent of trying the recommended stock settings first, because from what I can tell, norton tested enough to have gotten those spec's correct.
 
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comnoz said:
You may need to go richer on the needle jet. [or needle] The needle jet may be the full throttle restriction once the main is that large. If your running the MK3 needle that was designed for the full airbox that may be the problem.

Since you lowered the compression it would be normal to need larger jets.
Click to expand...

Higher compression usually needs larger jets. That is the reason you get more power - you use more fuel. The taper on the needle is usually enough to stop obstruction at full throttle, especially when using petrol as fuel. With methanol on 12 to 1 comp, the main jets are huge and the taper on the needle is usually extreme and sometimes the needle jets are recessed. . At 9 to I comp. with methanol , a petrol needle is used and the needle jets never need to be recessed. With petrol, you have none of that.
If the airbox has been removed, everything becomes leaner. But I would not have thought the taper on the needle would need to be changed. The taper on the needle compensates for the change in vacuum as the throttle is opened. Perhaps when the airbox is fitted, the motor sucks harder on the jets, so less taper is needed ?
 
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#300s are in or entering straight alcohol territory from what I have read; our Down-Under brothers will know more. I'd try the lean surge test first then, as Comnoz has advised, consider a larger needle jet. Fly cutting the float bowls is a good idea because it gives the fuel an opportunity to enter the bowl without foaming, but why, then, hasn't Amal done this years ago?? I'd really like to hear a good response; I have no idea. Do keep in mind that if you do this mod and cut too low your tuition payment could end up being a new carb(s)

Best
 
Flycutting the inlet can't hurt anything (unless poorly done as mentioned). A new bowl would be in order at the worst.

Since old bikes are rarely redlined for any length of time, I can see why Amal never did this. No idea if fuel flow is any better on MkII's or?
 
" #300s are in or entering straight alcohol territory from what I have read; our Down-Under brothers will know more. "

Rule of thumb is to multiply your petrol jet by 2.3.

ie a typical Commando 240 main jet would be replaced by a 550 or 560.


But still 300 mains are way too big for petrol on a street bike.
 
jimbo said:
interesting ... why are my plugs so clean?

I guess I could try leaner
Click to expand...

In that shot of the plug which shows down inside, the black ring is about 1/4 of an inch wide. It means the plug is not getting hot enough - mixture too rich. The black on the porcelain should burn off until the ring is about 1mm to 2mm wide. If the bike goes faster as you shut the throttle, it means the mains are too rich and the needles and needle are probably lean enough. Rich mixture makes the motor sluggish, too lean usually induces a cough, if it is needles. If you are too lean on the mains, you can easily start doing damage, It is normal to run the mains slightly rich.

I only read spark plugs when I am setting my main jets. Then I do high speed runs on full throttle, preferably uphill and do plug chops.
But most guys do not know how to read spark plugs. I only know because I once attended a lecture by Barry Smith, the works rider for Derbi. - Then it all made sense.
 
When I set the needles and needle jets, I rely on making the motor cough, then raise the needles one notch. If you do that with a two-stroke, you can cough-out the crank.
 
cash said:
An over advanced ignition can give the same symptoms.
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Found the ignition to be about +2* advanced at 2000 rpm , retarded it to 0, had intermittent backfires off idle, broke the starter drive, so I will be rebuilding the drive tomorrow. ( hope to have enough parts, sprag bearing?)I will degree wheel the timing at that time( loose timing scale?). But I have never had any trouble with this bike running funny or breaking the starter drive for over 20+ years. Rebuild it and all hell breaks loose.:( rally time closes in:(:(
 
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acotrel said:
If the bike goes faster as you shut the throttle, it means the mains are too rich
Click to expand...

Not exactly.

If closing the throttle from fully open to 7/8 open makes it briefly pull harder, that’s a sign of a weak main jet.

If doing that same thing makes the power dip and then slightly recover, then you may be too rich on the main jet.

Slightly closing the throttle from fully open enriches the mixture for a very short period of time, giving rise the the above effects.
 
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